The amounts of emissions, such as soot and NOx, generated in a combustion chamber of an internal combustion engine (in particular, a diesel engine) have a strong correlation with the state (in particular, temperature, fuel concentration, etc.) of gas mixture advancing within the combustion chamber (in particular, the state of the gas mixture after ignition). Therefore, in order to accurately control the generation amounts of emissions and reduce the generation amounts (discharge amounts) of the emissions, the state of the gas mixture must be accurately estimated.
There have been known various techniques for estimating the state of gas mixture. A fuel injection apparatus for a diesel engine disclosed in Japanese Patent Application Laid-Open (kokai) No. 2001-254645 estimates the temperature of gas mixture on the basis of various operation conditions, such as temperature of engine cooling water, temperature of intake air, and pressure of intake air, which affect the temperature of the gas mixture.
Incidentally, the state (temperature, etc.) of gas mixture advancing within the combustion chamber is successively determined depending on the temperature, etc. of cylinder interior gas, which change momentarily upon movement of the piston. Further, the state of gas mixture also depends on the degree of mixing of cylinder interior gas with fuel (accordingly, depends on, for example, excess air ratio, fuel concentration, etc. of the gas mixture). The excess air ratio of gas mixture is a value determined depending on the injection pressure of fuel contained in the gas mixture at the time of injection, and the density of cylinder interior gas at the time of injection. That is, the state of gas mixture is successively determined depending on the injection pressure of fuel contained in the gas mixture at the time of injection, and the density of the cylinder interior gas at the time of injection as well.
Meanwhile, the temperature, etc. of cylinder interior gas, which correspond to a time elapsed after fuel injection, change depending on a time point at which the fuel is injected. Further, the fuel injection pressure at the time of fuel injection, and the cylinder interior gas density at the time of fuel injection also change depending on the time point at which the fuel is injected. Accordingly, the state of gas mixture advancing within the combustion chamber changes depending on the time point at which the fuel contained in the gas mixture is injected.
In addition, when a certain quantity of fuel is injected into a combustion chamber, the fuel is continuously injected over an injection period set in accordance with the injection quantity. Accordingly, when a gas mixture advancing within the combustion chamber is microscopically observed, fuel contained in a portion near the front of the gas mixture has been injected earlier than fuel contained in the remaining portion. In other words, the injection time point of fuel contained in a certain portion changes depending on the position of the certain portion within a region occupied by the gas mixture (specifically, a distance from the front position of the gas mixture as measured along a direction toward an injection opening).
As can be understood from the above, when a gas mixture advancing within the combustion chamber is microscopically observed, the state (temperature, etc.) of the gas mixture (which corresponds to a time elapsed after fuel injection) changes depending on the position within the region occupied by the gas mixture. Accordingly, the degree of generation of emissions which are generated as a result of combustion of the gas mixture changes depending on the position within the region occupied by the gas mixture. In other words, since the state of the gas mixture advancing within the combustion chamber is not uniform, the degree of generation of emissions becomes non-uniform, so that the total generation amount of emissions generated within the combustion chamber is greatly influenced by such non-uniformity.
However, the gas mixture temperature (i.e., the state of gas mixture) estimated by the conventional apparatus is a value calculated under the assumption that fuel is injected at one time (instantaneously at a point in time at which injection is started), without consideration of the above-described injection period. That is, the gas mixture temperature is a value calculated without consideration of the above-described non-uniformity of gas mixture. Accordingly, if the total generation amount of emissions is estimated on the basis of the gas mixture temperature estimated by the conventional apparatus, the estimated total emission generation amount contains an error, raising a problem in that the generation amounts (discharge amounts) of emissions cannot be accurately controlled.